Much work has been directed to devising improved methods for forming hollow plastic articles and for uniformly distributing the plastic material therewithin. U.S. Pat. No. 3,757,718, which issued to H. G. Johnson on Sept. 11, 1973, for example, discloses a method of forming hollow articles from solid work-strengthenable plastic materials wherein a round blank of plastic is drawn into a cylindrical female die by a forming plunger from above and a cooperating axial plunger from below. Plastic between the forming plungers is thereby drawn out from beneath the lower end of the upper plunger to form the sidewalls of a container as the plunger is advanced into the female die. The cooperating lower plunger acts against the bottom of the plastic blank to control the outflow of material around the outer edge of the upper forming plunger.
U.S. Pat. No. 3,510,913, which issued to B. Edwards on May 12, 1970, concerns a method of manufacturing thin-walled thermoplastic containers with allegedly improved material distribution throughout the containers. The Edwards' method includes heating a web of thermoplastic material to its forming temperature, clamping the web between a forming mandrel and a female mold apparatus, axially advancing the forming mandrel into the female mold thereby forming an elongated intermediate article, then expanding the partially formed article against the inner wall periphery of the female mold. A vacuum within the axially advancing mandrel keeps the heated plastic from contacting the walls of the female mold until the expansion step, and also prevents undesired thinning of the plastic as it is stretched into its elongated preform.
A method for the differential heating and drawing of plastics is disclosed by U.S. Pat. No. 3,244,780, which issued to J. Levey et al. on Apr. 5, 1966. The Levey et al. apparatus includes an upper molding head platen having a centrally located disk portion constructed of a material substantially lower in heat conductivity than the balance thereof. This molding head platen is heated to a predetermined temperature and is lowered into a clamping relationship with a base platen which includes a female molding cavity axially aligned with the central disk of the molding head platen. Thermoplastic sheet material is clamped between two platens and the molding head platen serves to heat the thermoplastic sheet to a desired forming temperature. The portion of the thermoplastic material in contact with the central disk is not heated as much as the surrounding sheet material, thereby forming a cooler central area. Fluid pressure causes the thermoplastic sheet to form a hollow module as the central disk is axially retracted within the molding head platen, then the fluid pressure is reversed to expand the heated plastic material against the interior surfaces of the female cavity of the base platen. The base platen is kept at a cooler temperature and as the heated thermoplastic material comes into contact with the surfaces of the female cavity, further stretching is prohibited by rapid cooling of the material, with the cooler inner portion of the thermoplastic sheet being stretched last.
Another method for forming plastic articles from thermoplastic sheet material is shown in U.S. Pat. No. 4,088,718, which issued to R. F. Mulvaney, Jr. on May 9, 1978. Mulvaney, Jr. contemplates differentially heating a disk of thermoplastic sheet material and establishing concentric temperature zones to control expansion of preselected areas of the material during thermoforming procedures. The circular sheet material is heated such that its outer edge area is hotter than the central area. The heated disk is placed between a pair of opposed platens located between a forming plug and a female mold unit. The forming plug is then driven into the heated plastic sheet drawing it into the female mold unit. The opposed clamping platens are provided with means to adjust the clamping force thereby allowing the plastic sheet to be radially drawn inwardly as the forming plug pushes the plastic material into the female mold. Gradually the clamping means increase their pressure on the plastic sheet so that no part of the outer peripheral edge of the circular plastic disk can be drawn into the aperture of the female mold unit. The forming plug stretches the plastic material to a depth substantially adjacent the lower interior surface of the female mold unit, then pressurized air is introduced to expand the thermoplastic material against the inner surfaces of the female mold. The hotter areas of the plastic disk will expand first and contact the cooler walls of the female mold first, such cooler walls preventing further stretching of the plastic and thereby providing control of the resulting wall thicknesses. As in the Levey et al. process, the cooler central area is stretched last.
U.S. Pat. No. 4,420,454, which issued to Kawaguchi et al. on Dec. 13, 1983, describes a method of making a plastic hollow article quite similar to the Johnson method described above. Kawaguchi et al., however, requires that its plastic material stock be molecularly orientable, of substantially uniform thickness, and heated to a temperature below the upper limit of the molecularly orientable temperatures of the plastic material. Further, Kawaguchi et al. contemplates that the upper and lower forming plungers are to exert a relatively high compressing force on the plastic material stock as they move downwardly at the beginning stage of the forming operation to force the plastic radially outwardly, and thereafter reducing the compressing force to allow the plastic material to be drawn axially. Like Edwards, however, Kawaguchi et al. demonstrates that, as in all solid phase pressure forming methods known heretofore, substantially all the plastic which forms the sidewalls of a container must be obtained from the plastic stock material in the area below the forming plunger.
Despite all of the prior work done in the area of forming plastic articles, there remain problems and traditional limitations on the size and shape of plastic articles which can be formed therefrom. With these previously available forming methods, overall size and shape of a formed article was limited by the fact that effectively only the plastic beneath the plunger was available for the forming operation. Further, with the previously known methods, efficiency of plastic utilization was often forfeited in favor of more uniform wall thicknesses and the prevention of undue thinning in certain areas of the article. Even the use of intricate procedures and complex clamping/forming apparatuses have heretofore been unable to solve all of these problems.